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Government officials are poised to issue the first ‘heat health watch’ alert of the season before thermometers nudge 40C (104F) on Wednesday.

The warnings come into force when temperatures hit 30C (86F) during the day without falling below 15C (59F) at night.

With no sign of any cooldown until after the weekend forecasters predict another sizzling blast on Saturday, as these maps from The Weather Outlook show.

The Department for Food Agriculture and Rural Affairs (Defra) is warning of moderate levels of air pollution across most of the UK.

TheWeatherOutlook.Com

Another blast of roasting hot air is due to hit the UK on Friday

London is expected to be the hottest place as heat peaks, so Wimbledon has a 30 per chance of exceeding 36C

Marco Petagna

The Met Office said with very high pollen counts allergy sufferers face a double-whammy of misery.

Forecaster Marco Petagna said the South is braced to take the brunt of the heat although the mercury will rocket everywhere.

He said temperatures of 36C (97F) are likely in London and the south east with highs close to 30C elsewhere.

He said: “Things are starting to warm up on Tuesday with Wednesday and Thursday expected to be the hottest days.

“This is due to tropical continental air from Iberia and Spain, it is worth keeping an eye on the heat as it will build very rapidly and people may not be used to it.

“There is also the potential for pollution levels to start increasing and we are expecting very high pollen levels with grass and week pollen levels peaking at the moment.

“We will certainly get close to record temperatures this week, the record for July 1 is 34.8C (94.6F) and that could be reached.

“It is staying very warm into the weekend with another very warm spell on Saturday, after that temperatures will still be above-average.”

Netweather forecasters said with extreme humidity temperatures could feel close to 39.8C (104F) on Wednesday.

Clive James, training officer at St John Ambulance, warned this type of extreme heat can lead to exhaustion and heat stroke.

He said: “Extreme heat can be dangerous, particularly for the very young and old, but by being prepared you can spot the early warning signs and potentially be the difference between life and death in an emergency.

“Heat exhaustion and heat stroke are the most serious problems that can develop when the mercury soars so it’s essential that people can spot the signs, such as headache and dizziness, and get them somewhere cool and rehydrated as soon as possible.”

The first hints of what is to come arrived on Friday when temperatures in Writtle, Essex, soared to 27.7C (82F) - making it the hottest day of the year so far.

Thermometers hovered around the 25C (77F) over the weekend tempting sun-lovers out to parks and beaches.

N&S

At least the hot weather will be welcomed by some...

However, Leon Brown, forecaster for The Weather Channel UK, said tropical conditions will trigger violent thunderstorms later in the week.

He said: “The west is a little cooler on Thursday with a line of thunderstorms likely to run northwards over central Britain while the east of England remains hot and sunny.

“There is the potential for some local very heavy downpours over central and western England and flash flooding.

“Friday looks like another very hot, humid and mostly dry day over central and eastern Britain with temperatures again breaking 30C widely and could top 33C.

“There is a change from the west next weekend, but the heat may hang on across the east on Saturday.”

GETTY

The Government has advised anyone outside to drink plenty of water

James Madden, forecaster for Exacta Weather, said: “The upcoming week will see it becoming very warm to hot across many parts of the country.

“Wednesday could see temperatures peaking at 30C in parts of northern England, and parts of the South could record high temperatures in the region of 33-36C.

“The evenings will feel warm and muggy and night time temperatures could stay close to or above a sultry 20C in places.

“There will be the risk for some potentially heavy downpours and vigorous thunderstorm activity later in the week.”